Abstract
A flexible pipe consists of multiple unbonded layers. The annulus, between inner liner and outer sheath, consists primarily of pressure and tensile armour wires. These wires are made of carbon steel, which are cold rolled to pre-determined dimensions. The fatigue life of the whole pipe is to a large extent determined by the fatigue life of the tensile armour wires.
Fatigue testing has been done by dynamic, deflection-controlled, 4-point-bending, which is a reliable method to generate S-N curves where a multitude of wire samples over several stress ranges are tested. Tests have been done in different environments: air, deaerated synthetic seawater, and sour environment simulating annulus environments under different operating conditions.
Comparative tests of different armour wires in these environments form a foundation to discuss fatigue life in simulated annulus environments.
For two different grades of armour wires, it is shown that fatigue life varies when comparing air, deaerated artificial seawater, and sour (H2S) environments. The longest fatigue life is obtained in air followed by deaerated seawater with the sour environment having the lowest fatigue life. A method that qualitatively and quantitatively compares fatigue life at 100 MPa stress range is presented and comparative data given.
It is shown that for two different suppliers of the same grade of steel, the fatigue life is similar within each environment, air, deaerated synthetic seawater, and sour environment.
